This invention relates to a fluid bearing device for information equipment, audio/video equipment, office equipment and the like, and more particularly to a fluid bearing device that is best suited for a magnetic disk device (HDD) that are used in a notebook type personal computer, optical disk device, or fan motor etc.
An example of a prior fluid bearing device of this type is the HDD spindle motor shown in FIG. 2.
In this fluid bearing device, a sleeve 2 made of free-cutting brass is fastened to a cylindrical section 1a that is fixed upright to a base 1, and a shaft 3 made of stainless steel passes through this sleeve 2 such that it rotates freely. On the top end of this shaft 3 there is a hub 4 that is formed in an inverted cup shape and integrally attached to the shaft 3, and a dynamic fluid bearing is located between this shaft 3 and sleeve 2. A plurality of magnetic disks are placed at intervals in the axial direction on the outer peripheral surface of the hub 4.
A thrust plate 5 made of stainless steel in a disk shape is fastened on the bottom end of the shaft 3, and both of the flat surfaces of this thrust plate 5 are the thrust receiving surfaces 5s of the thrust fluid hearing S. And, on the bottom end surface of the sleeve 2, that is a mating member, faces the thrust receiving surface 5s on the top surface side of the thrust plate 5, and this bottom end surface of the sleeve 2 is a thrust hearing surface 2s of the thrust fluid bearing S.
Moreover, below the thrust plate 5, there is another mating member, that is counter plate 6, which is made of free-cutting brass and fastened to the base 1. The top surface of this counter plate 6 faces the thrust-receiving surface 5s on the bottom side of the thrust plate 5, and it forms the thrust-bearing surface 6s of the thrust fluid bearing S. Of the aforementioned thrust-receiving surfaces 5s and thrust-bearing surfaces 2s, 6s, at least the thrust-receiving surfaces 5s are formed with herring-bone-shaped or spiral-shaped grooves (not shown in the figure) for producing dynamic pressure, that are formed by etching, to form the thrust fluid bearing S.
Furthermore, a pair of radial-receiving surfaces 3r are formed on the outer peripheral surface of the shaft 3 with an interval in the bottom and top direction. Also, a pair of radial bearing surfaces 2r are formed on the inner peripheral surface of the sleeve 2 such that they face the radial-receiving surfaces 3r, respectively. In at least one of the pairs, the radial-receiving surfaces 3r, or radial bearing surfaces 2r, there are, for example, herring-bone-shaped grooves 7 for generating dynamic pressure, thereby forming radial fluid bearings R, respectively.
There is also a stator 8 fastened around the outer periphery of the cylindrical section 1a, and it faces the surface of the rotor magnet 9, that is fastened on the bottom side to the inner peripheral surface of the hub 4, by way of a gap therebetween to form the drive motor M, and the shaft 3 and hub 4 are rotated and driven together as one member.
As the shaft 3 rotates, a pumping action occurs in the grooves for generating dynamic pressure in both the thrust fluid bearing S and radial fluid bearing R, and dynamic pressure is generated on the lubrication oil in the bearing gaps of the fluid bearings S and R, and the shaft 3 is supported by but not in contact with the sleeve 2 and counter plate 6.
It is desired that HDD for recent notebook type personal computers, be thin as well as strong enough to withstand external impacts that could be applied during shipping or handling. In addition, it is also desired that they be low cost and durable.
As the HDD becomes thinner, there is a trend that the height of the fluid bearing device (spindle motor) used in the HDD becomes smaller, so it is necessary to increase the load capability for moment loads that act on the fluid bearing when swung. Therefore, it is necessary to design a larger bearing span for the plurality of radial bearings, however, since the height of the device is limited (for example 9.5 mm or less), it is necessary to decrease the thickness of the thrust plate.
However, when the thrust plate 5 is press fitted on the shaft 3 (shaft diameter is approximately 2.5 to 3.5 mm), a push-out force that is capable of withstanding an external impact of e.g. 1000G is necessary. However, since it is not possible to apply interference that would occur when performing press fitting with a stress that is greater than the yield stress of the material of the thrust plate 5, or interference that would have adverse effects on the flatness of both surfaces (thrust-receiving surfaces 5s) of the thrust plate 5, it is necessary that the thickness be 2 mm or greater even when the material of the thrust plate 5 is stainless steel, and since it is not possible to design a large bearing span at this thickness, it is difficult to increase the load capability for moment loads.
When the aforementioned interference is large, there is a possibility that the thrust plate 5 will be deformed during press fitting. This deformation is a bulging (convex deformation) in the center of the thrust plate 5, that is a portion formed with a hole for fitting on the shaft 3, which decreases the flatness of both surfaces of the thrust plate 5.
Moreover, as the thickness of the thrust plate 5 becomes thin, the portion that comes in contact with the outer peripheral surface of the shaft 3 in a fitting relationship becomes small (the length in the axial direction becomes shorter), so that it becomes easy for run out of the end surface (flat surface) of the thrust plate 5 to be large with respect to the center axis, and often it is not possible to satisfy the condition required for the bearing performance that the ran out of the end surface the no more than 2 xcexcm.
In other words, it is difficult to make the thickness of the thrust plate thinner in order to increase the load capability for moment loads, as well as satisfy the conditions required for the thrust plate such as the push-out force, flatness, and end surface run out.
On the other hand, in order to secure the reliability of the spindle motor operation, any contact between the outer peripheral surface 5a of the thrust plate 5 and the portion of the cylindrical section 1a that faces the outer peripheral surface 5a must be avoided, and in order to do that, the thrust plate 5 must be installed such that it is highly coaxial with the shaft 3. Particularly, in the case of a spindle motor of the fixed-sleeve, rotating-shaft type, any changes in the clearance between the outer peripheral surface 5a of the thrust plate 5 and the portion of the cylindrical section 1a that faces the outer peripheral surface 5a during rotation would cause unstable run out, and therefore it is extremely important that the thrust plate 5 is highly coaxial with the shaft 3.
Furthermore, when etching is adopted to form grooves in the thrust plate 5 for generating dynamic pressure, there is a problem of increased cost.
This invention is made to solve the problems with this kind of prior fluid bearing, and to provide a fluid bearing with high load capability for moment loads that would occur when the bearing device is swung even when the height dimension is limited, and in a low cost and with good operating reliability.